BRITISH FOSSILS. 



3 



are also present, forming more or less regular oblique rows of three, 

 sometimes of two, and with but slight traces of arrangement. The 

 tubercles become crowded towards the margin, and are thickly distri- 

 buted on the ventral surface, very regularly increasing in size towards 

 the mouth, immediately around which they are often fewer again and more 

 wide apart. Each tubercle is perforated on its summit, and has its base 

 borne upon an areolated boss* In very complete specimens, the margin 

 of the areola is surrounded by a ring of areolated granules, which, when 

 highly magnified, exhibit traces of perforations on their summits, as 

 represented in the plate. The spines are unknown. 



The apical disk is formed of five genital and five ocular plates. The 

 odd genital plate, that opposite the anal interambulacral area, is smaller 

 than the rest, and imperforate. The other four are conspicuously per- 

 forated for the oviducts. The left genital plate is larger than the rest, 

 occupies the centre of the disk, and is formed in part of the madre- 

 poriform tubercle, which in some specimens is very prominent and 

 always conspicuous. The eye-plates are small and pentagonal ; they 

 are, as usual, placed opposite the terminations of the avenues, and are 

 perforate at their lower parts, but still at some little distance from their 

 margins, by the eye-holes, which in this species are large and conspi- 

 cuous. [In Plate II., fig. 7, of Desor's Monograph of Galerites^ the 

 eye-holes of G. {Holectypus) depressus are represented as marginal, but 

 an examination of them in British specimens of that species has con- 

 vinced me that their true position is such as I now describe in G. 

 hemisphcericus. The madreporiform body is there described as distinct 

 from the ovarian plates, and the imperforated ovarian plate as largest 

 in the circle, both which statements are, as I interpret the structure, 

 errors.] The anus is placed in the lower and marginal half of the odd 

 interambulacral area ; it is pointed above, rounded below, so that its 

 general outline is somewhat pyriform, or rather drop-shaped. In what 

 may be considered normal examples, its widest diameter is about 

 equal to the distance between its lower margin and the mouth ; but 

 almost every specimen varies slightly in this respect, and the result of 

 an examination of more than fifty specimens has convinced me that, 

 further than the constant marginality of the anus, the exact proportion 

 its lower part bears to the ventral disk is not to be received as a specific 

 character. Its breadth also varies in different individuals, in all other 

 respects exactly similar. 



Varieties. — Variations in convexity and in proportion of breadth to 

 height are very conspicuous in this species, and among a number of 

 specimens from any one locality very few will be found in this respect 

 exactly alike. The other points of variation are the distance between 

 the anus and the mouth, and the number and regularity of the tubercles. 



[ill. vi.] B 2 



